Sheathed heating units of the type arranged in flat convoluted loops or coils are generally arranged on a support grid consisting of a plurality of radially disposed support arms. In some prior art units, each convolution of the heating coil has been anchored to each support arm. This is usually accomplished by means of upwardly extending pairs of tabs that are formed on the support arms. Each convolution of the heater is positioned between the tabs which grip the sheath of the heating element. This method of securing the heating coil does provide a unit that is free from vibration noises; however, it does not permit movement of the sheathed heater relative to the support grid as they expand and contract with temperature changes. This lack of movement between the suppot arms and heater creates stresses that can ultimately lead to heating unit failure. Further, this method also results in a relatively large amount of heat dissipation through the grip support structure and also results in distortions at each point of anchor between the heater and support arms that can lead to premature failure of the sheathed heating unit.
In other prior art units, the convoluted coil is secured to the support grid at selected locations. For example, in some instances, only the outer pass or convolution of the heating element is anchored to one of the support arms. Except for this single point of attachment of the heating element to the support grid, the convolutions of the heating element are free to move on the upper edges of the supporting grid as they expand and contract with temperature changes. While this arrangement solves the problem relating to heat dissipation through the support assembly, it does however result in noise created by vibrations between the sheathed heating unit and the support structure. These vibrations and, more particularly the resulting noise, are objectionable.
The combination of relatively thin vertical spider or arm members for supporting the heating element together with staking the heating unit at only one location of one arm presents a minimum heat sink potential and, accordingly, a relatively efficient heating unit. However, the unsupported heating element results in some objectionable noise levels generally caused by vibrations between the heating unit and the spider arms.
Accordingly, it is an object of the present invention to provide a sheathed heating unit that is efficient in operation and relatively free from noise caused by vibration between the heating coil and support structure.